Just recently I noticed on my Nexus 7 that when I start some apps - and it's not just those allows to have superuser rights - I can briefly see a "red frame" all around the screen area of the app that goes away in a second or maybe a little less.

Does anyone know where this comes from and in particular what it means?

Note: as you probably guessed from the contents, my device is rooted. Also note that it's well possible that one of the installed apps is responsible for this, yet I don;t know which. But I'd like to know.